The present invention relates to a rolling bearing and, more in particular, it relates to rolling bearings used in circumstances undergoing high temperature, large loads and large vibrations, for example, ball bearings for use in transmissions, hub units and engine auxiliaries (such as alternators, intermediate pulleys and solenoid clutches) of automobiles, guide roll bearings and backup roll bearings for use in iron and steel making, or rolling bearings suitable to roller bearings such as those used in railway vehicles, as well as a method of working rolling elements of them.
In the calculation for the life of rolling bearings, it has been known to determine a basic dynamic rating load of bearings while assuming that a probability for the occurrence of flaking in two objects in contact with each other (bearing ring and rolling element) such as a fixed ring and a rolling element, and a rotational ring and a rolling element is identical between both of them, and combining a basic load capacity of the rotational ring and the basic load capacity of the fixed ring (for example, in xe2x80x9cDynamic Load Capacity of Rolling Bearing Roller Bearingxe2x80x9d written by Junzo Okamoto, printed from Seibunsha Co. Ltd.).
On the other hand, as one of countermeasures for improving the life of rolling bearings, cleanliness has been improved for the material of the bearing ring, and non-metal inclusions in steels have been decreased to greatly improve the rolling life in recent years (NSK Technical Journal, No. 652 (1992), Pages 1-8).
In the same manner, cleanliness has also been improved in the coil materials used for the rolling element to improve the reliability. However, the probability for the presence of non-metal inclusions at the central portion of the coil material tends to be higher compared withoursolling bearings not using the central portion of bar materials. Accordingly, in a circumstance where bearings undergo large loads and large vibrations, early flaking may sometimes occur in the rolling element.
As the prior art for improving the indentation resistance and the rolling life of the rolling element, there has been a technique, as described in Japanese Patent Examined Publication No. Hei 1-12812, of applying quenching and tempering to a rolling element and then applying a mechanical surface hardening treatment by air jet peening to cause plastic deformation to the surface layer thereby obtaining a large residual compressive stress layer to improve the fatigue life and improve the hardness thereby reducing the occurrence of surface flaws during handling of the rolling element.
Further, as a technique for increasing the life of steel balls for grease-sealed bearings, there has been reported, as disclosed in Japanese Patent Unexamined Publication No. Hei 3-173714, a technique of controlling the residual tensile strength in the direction of the thickness in the surface layer to 150 MPa at the maximum and, preferably, from 50 to 150 MPa in a case where a work hardening treatment is applied to the surface of a steel ball to result in a difference of hardness of HOURSC 1 or higher between the inside and the surface layer of the steel ball, thereby suppressing intrusion and accumulation of hydrogen into tensile stress exerting regions to prevent occurrence of fatigue cracking and flaking.
Further, as balls for use in ball bearings and a manufacturing method thereof, Japanese Patent Unexamined Publication No. Hei 6-264929 discloses a prior art of applying a tempering treatment after the surface hardening treatment for preventing aging deterioration of the accuracy on the surface of the ball thereby preventing aging deterioration of the acoustic characteristics of the ball bearings.
However, in a case of use in a working circumstance under large loads and large vibrations, since a sort of plastic working corresponding to contact fatigue such as mechanical surface hardening by air jet peening has already been applied to a rolling element as shown in Japanese Patent Examined Publication No. Hei 1-12812, excess plastic deformation of the surface layer proceeds in an accelerated manner in a working circumstance where the rolling element undergoes similar large loads and large vibrations and, as a result, this tends to cause early flaking.
Further, as disclosed in Japanese Patent Unexamined Publication No. Hei 3-173714, in the case of the technique of suppressing intrusion of hydrogen into the tensile stress exerting regions by controlling the maximum value of the residual tensile stress in the surface layer, to prevent occurrence of fatigue cracking and flaking, the maximum value of the residual tensile stress occurs near the position for the maximum shearing stress and, accordingly, propagation of cracks is further accelerated under large loads and large vibrations in a state where the maximum residual tensile stress of 150 MPa is loaded, so that the effect of extending the life can not be expected.
Further, in the technique disclosed in Japanese Patent Unexamined Publication No. Hei 6-264929, for coping with the acoustic problem caused by aging deterioration of the accuracy on the surface of a ball for use in HDD when used in a circumstance under small load and high speed rotation, rough grinding is applied after quenching/tempering treatment and, further, a tempering treatment is applied again after the surface hardening treatment followed by finish grinding. It takes no consideration for relieving the residual work strain which is most important for flaking under large loads and large vibrations that is the problem in the present invention, to still leave a room for improvement in this respect.
Furthermore, Plasticity and Working (Journal of Japanese Plastic Working Society), vol. 39, No. 446 (1988-3) shows, in xe2x80x9cSeveral Problems for the Manufacture of Ballsxe2x80x9d, that grinding for correcting the shape of a steel ball (ball) and peening for improving the strength are required regarding the effect of the residual stress on the fatigue life but excess working may deteriorate the fatigue life, and states that the subject resides in the optimal peening.
The present invention has been accomplished for overcoming such drawbacks in the prior art and it is an object thereof to obtain a rolling element with improved resistance to rolling contact fatigue while keeping the hardness by devising a combination of a mechanical surface hardening treatment and a heat treatment to a rolling element of a rolling bearing and to provide a rolling bearing of long life even under large loads and large vibrations, by incorporating the rolling element described above.
More specifically, it intends to obtain a rolling element improved with the indentation resistance and rolling contact fatigue resistance by controlling the residual stress on the surface of a rolling element and, optionally, defining a relation with a residual stress value at a position for 2% diameter depth below the surface to optimize the residual stress distribution, thereby providing a rolling bearing having a long rolling life even under large loads and large vibrations.
Present invention relates to a rolling bearing capable of attaining such an object and a method of working a rolling element thereof.
A rolling bearing of the present invention has a feature in a rolling bearing which is used with a plurality of rolling elements being disposed between a fixed ring and a rotational ring, wherein the amount of residual austenite in the surface layer of the rolling element is from 0 to 15 vol %, a final residual compressive stress value is from xe2x88x92600 to xe2x88x921200 MPa and a working strain relieving degree is from 10 to 60%.
That is, the rolling element is manufactured from a coil material and, in a case of the coil material, since a probability for the presence of non-metal inclusions in the central portion is higher than that in the bearing ring, flaking tends to occur particularly in the rolling element. In view of the above, it is effective for extending the life to prevent occurrence, particularly, of rolling contact fatigue cracking or flaking by controlling the residual stress in the rolling element as described above.
In this case, numerical values are defined by the reasons described below. When the amount of the residual austenite in the surface layer of the rolling bearing exceeds 15 vol %, the indentation characteristic is deteriorated tending to cause flaws on the surface. Further, when the final residual compressive stress value at the surface of the rolling element is less than xe2x88x92600 MPa, the effect of suppressing the crack propagation is reduced and, on the other hand, large residual tensile strength exerts in the radial direction when it exceeds xe2x88x921200 MPa. Therefore, in each of the cases, crack propagation leading to flaking is promoted. Further, if the working strain relieving degree is less than 10%, the plastic strain generated by excessive working can not be removed sufficiently and, on the other hand, the effect of the applied residual compressive stress is eliminated if it exceeds 60%, to deteriorate the performance of the flaking resistance.
In the rolling bearing according to the present invention, in the value for the residual compressive stress ("sgr"D) at the depth of 2% Da as the ratio to the diameter of the rolling element from the surface of a completion product of the rolling element is xe2x88x92400 to xe2x88x921000 MPa.
Further, the residual stress distribution can be optimized by defining a relation between a final residual compressive stress value "sgr"S on the surface of the rolling element and a residual compressive stress ("sgr"D) at a depth of 2% Da as a ratio to the diameter of the rolling element from the surface of the completion product so as to satisfy: |"sgr"S|xe2x89xa7|"sgr"D|, and a range of |"sgr"D|=400-1000 MPa.
This is because the rolling contact fatigue strength is improved by optimizing the residual stress value "sgr"D at a position for 2% depth to the diameter below the surface of the rolling element within a range from xe2x88x92400 to xe2x88x921000 MPa which is lower than the residual compressive stress value as on the surface. If the residual stress value "sgr"D for 2% Da is less than xe2x88x92400 MPa, propagation of cracking can not be suppressed sufficiently and, on the other hand, if the value exceeds xe2x88x921000 MPa, the residual stress caused by excess working is not relieved sufficiently tending to proceed the plastic deformation by rolling contact fatigue to shorten the life. While the range for "sgr"S and "sgr"D is selected as described above, since a relation: "sgr"Dxe2x89xa7"sgr"S can also exist in addition to the relation as "sgr"Sxe2x89xa7"sgr"D, it is indicated by absolute values.
The present invention also relates methods of working a rolling element of rolling bearings.
The method of working a rolling element of a rolling bearing according to the present invention comprises a quenching/tempering step of applying hardening by heat treatment after quenching and tempering a rolling element made of a bearing steel, a mechanical working step of applying plastic deformation to the surface for providing a residual compressive stress on and below the surface of the rolling element after the quenching/tempering step, a secondary tempering step of relieving plastic strain after the mechanical working step, and a finishing step of finishing the surface of the rolling element to a predetermined size.
That is, the surface hardening treatment is applied to the rolling element made of the bearing steel after quenching and tempering and then the tempering treatment is applied again to relieve the working strain of the surface layer by the residual compressive strength on the surface and finish grinding is applied to improve the rolling contact fatigue under large loads and large vibrations for keeping the hardness of the surface layer.
In this case, it can be defined such that a relation between the residual compressive stress ("sgr"1 MPa) on the surface of the rolling element caused by plastic working strain in the mechanical working step and a residual compressive stress ("sgr"2 MPa) on the surface of a completion product after the finishing step satisfies: {("sgr"1xe2x88x92"sgr"2)/"sgr"1}xc3x97100%=10-60% and provision of the plastic working strain in the mechanical working step and relieving of the plastic strain in the secondary tempering step are applied.
Then, it can be defined that the value of "sgr"1 is: "sgr"1 =xe2x88x92900 to xe2x88x921500 MPa and the value of "sgr"2 is :"sgr"2=xe2x88x92600 to xe2x88x921200 MPa.
Further, it can be defined that the amount of residual austenite on the surface of a rolling element after the quenching/tempering step is : xcex3R=10 to 30 vol %, and the amount of residual austenite on the surface of the rolling element after the finish working step is : xcex3R=0 to 15 vol %.
Further, more specifically, a more preferred result can be obtained by defining that heating at 830 to 870xc2x0 C., oil cooling and tempering at 130 to 160xc2x0 C. are conducted in the quenching/tempering step and tempering at 150 to 240xc2x0 C. is conducted in the secondary tempering step.
In the rolling element for the rolling bearing of the present invention obtained by the working method described above, the final residual compressive stress ("sgr"1 MPa) on the surface of the completion product of the rolling element satisfies a relation: |"sgr"S|xe2x89xa7|"sgr"D|and |"sgr"D|=40 to 1000 MPa, when indicated by the absolute value for the residual compressive stress ("sgr"D MPa) at the depth of 2% Da as a ratio to the diameter of the rolling element from the surface of the completed product.
In the working method for the rolling element according to the present invention, the mechanical working step comprises charging quenched/tempered steel balls for use in a predetermined rolling bearing in a space volume of a rotating barrel box and applying plastic working strain to the surface of the steel balls by collision between each of the steel balls.